Mammalian in vivo and in vitro systems have been exposed to biomedical ultrasound (2.2 MHz, .5 to 30 W/cm2) for up to 5 min. The in vivo exposures utilized partially hepatectomized rats whose remaining livers were sonicated; the sonicated and sham rats showed no differences in mitotic index of regenerating liver cells. The in vitro experiments utilized HeLa cells which showed (1) an ultrasound intensity related immediate cell lysis and (2) an ultrasound intensity related decrease in percent intact viable cells. These latter results indicate that ultrasound cannot only lyse cells, and of the remaining intact cells, their viability is also related to the intensity of the ultrasound. The likelihood of there being lethal and sublethal effects on cells induced by biomedical ultrasound seems likely. BIBLIOGRAPHIC REFERENCES: Kaufman, G.E., Miller, M.W., and Carstensen, E.L. Effects of biomedical ultrasound on animal systems; abstract. Ultrasonic Biophysics Meeting, 8 April 1976, Washington, D.C.